EPSC Abstracts Vol. 6, EPSC-DPS2011-553, 2011 EPSC-DPS Joint Meeting 2011 c Author(s) 2011

Characterizing exoplanetary atmospheres through infrared polarimetry

R.J. de Kok(1), D.M. Stam (1) and T. Karalidi (1) (1) SRON Netherlands Institute for Space Research, Utrecht, the Netherlands, ([email protected] / Fax: +31 (0)887775601)

Abstract of temperature or clouds, or a non-spherical shape of the object can cause the thermal radiation to have a We present calculated thermal polarization signals net polarization signal. For polarized Brown Dwarfs a from hot exoplanets, using a doubling-adding radia- plausible candidate for the source of polarisation is a tive transfer model that spatially resolves the disk of flattening of the body due to fast rotation [4]. Here, we the planet, allowing the simulation of horizontally in- calculate signals of horizontally inhomogeneous plan- homogeneous planets. Our calculations show that the ets and explore parameters that determine the strength degree of linear polarization, P , of an exoplanet’s of these thermal polarization signals, and discuss the thermal radiation is expected to be highest near the value of infrared polarimetry for the characterization planet’s limb and that this P strongly depends on the of exoplanet atmospheres. temperature and its gradient, and the scattering proper- ties and distribution of the cloud particles. Planets that 2. Spatially resolved polarimetry do not appear to be spherically symmetric, e.g. due to flattening, cloud bands or spots in their atmosphere, We calculate the radiative transfer in a locally plane- differences in their day and night sides, and/or ob- parallel, vertically inhomogeneous planetary atmo- scuring rings, give P values that are often larger than sphere fora range of emission angles using a doubling- 0.1%, in favorable cases even reaching several percent adding method [6], which fully includes all orders of at near-infrared wavelengths. Detection of thermal po- scattering and polarization. We first look at how the larization signals can give access to planetary param- polarization varies across the planetary disk with vary- eters that are otherwise hard to obtain: it immediately ing atmospheric parameters. We assume ad hoc tem- confirms the presence of atmospheric clouds, and P perature profiles with temperature gradients that are can then constrain atmospheric inhomogeneities and constant with ln p and a geometrically thin cloud layer, the flattening due to the planet’s rotation rate. For zon- high in the atmosphere, with a given optical depth and ally symmetric planets, the angle of polarization will Rayleigh scattering particles. yield the components of the planet’s spin axis normal We find that P is largest near the limb of the planet, to the line-of-sight. Our calculations also show that P where singly scattered light from below is scattered is generally more sensitive to variability in a cloudy at angles close to 90◦, which gives the highest polar- planet’s atmosphere than flux is, making polarimetry ization for Rayleigh particles. For an atmosphere that very suitable for detecting phenomena associated with has temperatures decreasing with decreasing pressures atmospheric dynamics. near its limb, the polarization direction near the limb is parallel to the local horizon. A larger temperature 1. Introduction gradient will give rise to a larger P at the limb, as the radiation field near the scattering particles will become Stellar light that is reflected by a planetary atmosphere more dominated by the hot radiation coming from di- will usually be polarized, due to scattering of clouds rectly below, before being scattered at angles near 90◦. and molecules [3] [5]. On the other hand, light be- For a negative temperature gradient (temperatures ris- ing emitted by, for example, a Brown Dwarf can also ing with decreasing pressure) multiple scattering is become polarized when scattering takes place in the more important, resulting in the polarization direction atmosphere [2]. For a spatially unresolved object the being orthogonal to the local horizon. Because of the net degree of linear polarization, P , of the thermal ra- multiple scattering, P is lower for temperature profiles diation will equal zero when it is horizontally home- with negative gradients than for positive ones. geneous. A horizontally inhomogeneous distribution Increasing cloud optical thickness for a positive temperature gradient will at first increase P near the will show little variability. On the other hand, if the limb, but will decrease again when the optical thick- polarization is caused by transiting clouds or hot spots, ness becomes larger than 0.1, as multiple scattering both P and the polarization direction will vary in time, starts playing a larger role.∼ Lowering the cloud layer and stronger than flux. Hence, a time series of polar- will not change P much until the gas absorption opti- ization measurements will give insight into dynamical cal thickness will be similar to the cloud optical thick- processes, and a periodic signal could even yield at- ness, when P will vanish as absorption starts dominat- mospheric rotation rates. ing. When the cloud is at the top of the atmosphere, P There are several reasons why planets like those in will be largest in gas absorption bands, whereas if the the HR 8799 system might be more prone to produc- cloud is at altitudes where gas absortion is significant, ing polarized signals than Brown Dwarfs, which have P will be lowest in absorption bands. been shown to be polarized by up to a few percent. Firstly, the surface gravity of planets is lower than that 3. Disk-integrated polarimetry of Brown Dwarfs, making them more flattened for a given rotation rate. Secondly, broadband flux mea- We define a latitude-longitude grid of 2◦ 2◦, calcu- surements suggest the HR 8799 planets to be more × late the polarization signals across the disk, and inte- cloudy than Brown Dwarfs, which might be common grate them over the disk. Horizontally homogeneous to young planets [1]. Thirdly, the effective tempera- spherical planets indeed yield a net P of zero. For tures in the planets’ atmospheres are lower than those a horizontally homogeneous ellipsoidal planet and a in known polarized Brown Dwarfs and, given a certain temperature slope of 300 K/ln p, we get a similar val- temperature gradient, lower temperatures will yield ues of P as a function of oblateness as [4] for their higher values of P . models with Teff =1800 K and log g = 4.5. We also calculated P for planets with a banded structure and for planets with obscuring rings under a wide range Acknowledgements of viewing geometries. The net P is often larger than 0.1%, in several cases reaching several percent. The This work is supported by the Netherlands Organisa- above three cases all have a zonal symmetry, which tion for Scientific Research (NWO). We thank Wiel means that the axis of symmetry is alligned with the Wauben making his scattering code available to us. spin axis of the planet. In these cases, the direction of polarization is either parallel or orthogonal to the spin axis of the planet. Calculations of a dusty hot spot ro- References tating into view, as well as calculations of planets with [1] Madhusudhan, N., Burrows, A., & Currie, T. 2011, a day-night temperature and dust difference show that Arxiv e-print 1102.5089 P and the direction of polarization vary more strongly than the infrared flux. [2] Ménard, F., Delfosse, X., & Monin, J. 2002, A&A, 396, L35

4. Summary and Discussion [3] Seager, S., Whitney, B.A., & Sasselov, D.D. 2000, ApJ, 540, 504 Our calculations show that the infrared radiation emit- ted by exoplanets can be polarized by several percent [4] Sengupta, S. & Marley, M.S. 2010, ApJ, 722, L142 and that the polarization is sensitive to the temperature, [5] Stam, D.M., Hovenier, J.W., & Waters, L.B.F.M. 2004, the temperature gradient, the cloud particle properties, A&A, 428, 663 cloud height and the cloud optical thickness. These high values of P can occur both for ellipsoidal plan- [6] Wauben, W.M.F., de Haan, J.F., & Hovenier, J.W. 1994, ets (like for Brown Dwarfs [4] ) and horizontally in- A&A, 282, 277 homogeneous planets. A detection of an infrared po- larized signal will immediately confirm the presence of scattering particles, such as cloud particles, in the planetary atmosphere. In addition, the polarization di- rection will reveal the components of the planet’s spin axis normal to the line-of-sight for zonally symmet- ric planets. For such planets, both P and the intensity